Baby fine flat lifeless hair here too.
After trying every expensive brand of shampoo and product, I find the cheaper things work better for me.
I find mine looks thicker and fuller the more I abuse it!...so swimming in sea water and chlorinated indoor pools (swells the hair shaft?), though it’s not a realistic solution for me to suggest anyone else spend hours in the pool for thicker hair!
Hairdresser suggested thin foil highlights as they similarly swell the hair shaft so it looks thicker and with more body.
For years I have had success sticking with a super cheap product, normally around £1 in those high street shops, it’s a quality German brand - will use it forever;
Schwartzkopft Gliss Hair Repair Liquid Silk - shake before using and just a single spray on the ends, a single bottle lasts a year. Wouldn’t be without this, also get lots of comments about shiny hair.
Also echo the recommendation for biotin supplements, but longer term, not a quick fix.
Never had dandruff, but accidentally found that the cheapest supermarket dandruff shampoos work well to make my hair look and feel thicker too.
Reading all the replies with interest and more ideas for me to try.